Of all the air to air weaponry developed in the latter half of the 20th
century, the air to air guided missile has probably had the greatest
impact, affecting the design of weapon systems, airframes, propulsion
and often leading to a complete reassessment of combat tactics.

Air to air missiles (AAMs) differ principally in guidance,
the two broad groups being radar guided and heat-seeking or infra-red
(IR) missiles. Of the two categories, the second, by virtue of it's
simplicity and lower demands on launch aircraft complexity, has become
the numerically superior, arming high performance fighters like the
F-15
or F-14 operated by frontline air forces, yet also equipping vintage
1950's fighters, defending Third World countries.

The initial deployment of heat seeking missiles began in the
late 1950's, with the USAF acquiring it's first AIM-4 Falcons and AIM-9
Sidewinders, however, it was not until the Viet-Nam war that the AIM-9
saw widespread use. The weapon was not as successful as expected,
reliability was a particular problem, especially with the USN (repeated
carrier launches and recoveries - quote ''banging them on the deck
after
every flight''), but when the missile did work, it was effective, which
may be confirmed by a number of NVAF pilots who had the unique
experience of a 'Winder entering the tailpipe of their sturdy Mikoyan,
resulting in it's subsequent bisection.

Current versions of the AIM-9 are a vast improvement, though
they are to be replaced by the end of this decade by the ASRAAM - solid
state electronics allow for higher reliability and enable the guidance
to be ''smarter'' in between discriminating targets and resisting
jamming.

As their name implies, heat-seeking missiles home on to the
hot areas of a target. The target will usually both reflect and emit
infra-red radiation, which propagates through the atmosphere, losing
it's intensity due to number of effects. This radiation is detected by
the missile's seeker head, which, if the conditions are right, will
then
provide the guidance with the relative position of the target enabling
the weapon to home in and destroy the target. In order to fully
appreciate the problems involved in creating an effective weapon of
this
kind, we must examine the behaviour of infra-red radiation, the
characteristics of an aircraft as a source of IR energy, the manner in
which this energy travels through the atmosphere and finally, how the
missile seeker processes it to gain information as to the target's
position.

Infra-Red Radiation

The infra-red is a term used to describe a particular group
of
electromagnetic waves, those which are longer than visible wavelengths
and shorter than microwaves, numerically the band between 0.8 and
1000.0
micrometres. This means that infra-red radiation has very many
properties similar to visible light, it can be focussed or diffused,
absorbed or reflected.

The reason why the IR is so important is that it may be
closely associated with heat and it's transfer from bodies. When we
heat
an object, or a volume of material, we are feeding energy into it - at
an atomic level this energy is in the form of crystal lattice
vibrations
(probably the best way of visualizing the process is to picture an
infinite lattice of tiny balls, each connected to it's neighbours by a
spring, imagine then what occurs if we tap one of the balls) every atom
vibrating about some equilibrium position in the lattice.

To make things more complicated, in the physical world, the
amount of energy each atom can transfer to it's neighbour is limited to
an amount hf (h is Planck's constant and f is the frequency of
vibration) which leads to an interesting result - a heated body emits
radiation throughout a continuous band of wavelengths, the relative
amplitudes (levels) of each wavelength depending on the body's
temperature.

Graph 1. illustrates this relationship. This means that every
object radiates energy, the dominant wavelengths depending on the
temperature. As it turns out, objects at temperatures around and below
1200 C radiate mainly in the infra-red band, e in the higher the
temperature, the shorter the dominant wavelengths. (note: IR radiation
can also be generated by exciting a molecular gas, as the frequencies
at
which the molecules rotate and vibrate fall into the IR band - this
type
of radiation forms narrow bands in the spectrum -see TE Dec.1981,
Lasers).

Sources of IR energy

The easiest answer to that question is - any warm or hot
object- however, the subject deserves a little more attention. The
single greatest source of IR radiation is the sun, however only part of
this energy would be available to illuminate the Earth's surface, as
large amounts are absorbed and scattered in the atmosphere,
particularly
by clouds and moisture. The Earth's surface becomes a secondary source
of IR, as it is being bombarded both with visible and IR energy from
the
sun and as a result, is being heated.

Any processes releasing heat also lead to the emission of IR.
All heat engines ie the internal combustion engine or jet turbines
radiate IR from various parts of their structures and also release hot
exhaust gases. All warm blooded creatures emit IR.

As far as a guidance system is concerned, all IR radiation
from sources other than the target is a nuisance - background clutter
which will lower detection ranges or even swamp the emissions of the
target. Fortunately most of the IR energy emitted by the Earth's
surface
falls into the vicinity of 10 micrometres, whereas the Sun's radiation
peaks in the visible band and reflected off the Earth's surface would
tend to swamp the region above 3 micrometres, hence leaving a window
around 4 micrometres. The sky itself reflects and scatters a certain
amount of IR, though it's intensity is lower than that of the Earth's
surface.

Graph 1. Relative emitted
power
versus wavelength of emitted radiation for an ideal blackbody radiator.
These curves illustrate the relative amounts of IR energy emitted at
various wavelengths for varying temperatures, the dominant wavelengths
can be seen as becoming shorter with increasing temperatures. (Note: an
ideal blackbody by definition radiates equally well at all wavelengths,
an aircraft tailpipe is close to a blackbody in a very limited range of
wavelengths).

Diagram 2. The Infra-red
signature
of a fighter aircraft. Here a likely target for a Sidewinder, a
MiG-23BM Flogger powered by a 25,000 lb R-29 afterburning turbofan.

The aircraft itself both reflects IR from the sun and emits IR from
it's hot parts, particularly the afterburner nozzle. The exhaust plume
temperature curves illustrate sections through areas of equal
temperature, the upper half with lit afterburner, the lower on dry
thrust. The lower temperature (cca 100 deg) curves for the afterburning
mode extend to a distance greater than 100 metres behind the aircraft.
The plume of a turbojet on dry thrust is narrower and hotter than taht
for a turbofan, which mixes cool bypass air with the turbine exhaust
gasses.

An Aircraft as a Source
of IR

Modern military aircraft are, in spite of the efforts of
their
designers, abundant sources of IR energy. The principal heat source is
the propulsion, as jet engines have an efficiency far below 100%, a
considerable amount of energy is thrown away, advertising the
aircraft's
position.

The most intense IR source in a jet aircraft is the tailpipe
(afterburner off). The exhaust gas temperature (EGT) of a typical
turbojet, ie the J79, is around 950 deg C, newer engines like the F100
have an EGT around 1300 deg C. The highest intensity is thus for
wavelengths around 2 to 3 micrometres (for the physics oriented, the
tailpipe is modelled as a blackbody, or rather greybody radiator).

The exhaust gases leaving the tailpipe form a plume, as they
expand and cool. On dry thrust, the tailpipe is the strongest radiator,
the plume being cooler, particularly in the instance of high bypass
ratio turbojets (F404) or turbofans (F100), where the turbine exhaust
gases are mixed with bypass air from the fan.

Lighting the afterburner causes further radiation of IR, in
fact the exhaust plume, around 2000 deg C, then dominates the
aircraft's
signature, being hotter and physically larger than the tailypipe.
(note:
at speeds above 2.5M the plume radiance will decrease due to the
decreased overall engine pressure ratio).

Aside from tailpipe/plume emissions, the hot parts of the
engine eg exterior of afterburner nozzle, also radiate. High speed
flight will heat the aircraft's skin and the engine will usually heat
up
parts of the airframe .

A further source of emissions is reflected sunlight/IR,
conventional paints apparently reflecting around 60%, though the newer
low IR greys (USN F-14, F-4, F-18 etc.) reflect around 5 to 15%. A well
polished canopy may also reflect enough energy for a lock-on.

From the practical point of view, the IR signature of an
aircraft is impossible to eliminate, the best one could ask for is a
reduction. The use of turbofans reduces the overall EGT and where
possible, parts of the airframe may be used to shield the exhaust, as
in
the A-10, where the tail surfaces screen off the relatively cool
exhausts of the TF34s (note: the positioning of the engines makes it
impossible to gain a lock-on with a shoulder launched SAM, eg SA-7,
until the aircraft has covered a relatively large distance, assuming
the
aircraft passes over the launch site. ) The use of low reflectance
paints and flat canopies (helos) can be of some use.

Atmospheric Propagation
of IR Radiation

The atmosphere basically affects IR energy in three different
ways - absorption, scattering and scintillation (for a more detailed
treatment see TE Dec.1981, Lasers).

Photons of IR wavelengths are absorbed mainly by atmospheric
carbon dioxide and water molecules, fortunately for the militarist,
this
is a quantum physical effect and is confined to particular bands in the
IR, leaving transmission "windows".

The most important windows in the near IR are centred on 1.6
micrometres, 2.2 micrometres and 3.75 micrometres, the last being the
widest at about 1 micrometre, at an altitude cca 5000 m and low water
vapour concentrations these allow for up to 95% transmittance at ranges
cca 30km (16.5 NMI). If the water vapour concentration increases,
scattering becomes noticeable. Scattering occurs when the wavelength of
the IR is comparable in size with the scattering particles. Clouds and
fog contain droplets around 1 micrometre in size - this results in
extremely low transmission throughout most of the IR band.

On the other hand, rain droplets are much larger, with the
seemingly surprising result that IR transmission through rain is
substantially better. Rain is liable to degrade the systems
performance,
but still allow it to function (at 1.8km / 1 NMI transmittance for
light
rain is cca 90% and for heavy rain cca 65%).

Scintillation is caused by the same effect responsible for
the
blooming of laser beams, local variations in the atmosphere's
refractive
index caused by variations in temperature (eg observable flickering of
distant images on a well heated bitumen road). This effect isn't
particularly important for guidance systems, as the apparent changes in
the target's position get smaller and smaller as the weapon approaches
the target, once the angular size of the target becomes larger than the
size of the apparent changes in position, it can be neglected.

The effects of the atmosphere can be summarized as a lowering
of the target's intensity over a distance, and the introducing of small
position errors at large distances. These two effects basically serve
to
limit the maximum range at which a guidance system may detect, track
and
lock on to a target.

The Heat-seeking Missile

The aircraft as an intrinsic source of IR energy and the
reasonably good propagation of IR clearly indicate the potential for
relatively simple, accurate short range missile guidance. As the target
itself emits all the energy needed for detection and guidance the
weapon
may be fire-and-forget, without the need for complex and cumbersome
fire
control and illuminating radar. As a relatively simple system, the
weapon may be smaller and lighter, it's fire-and-forget ability makes
it then ideal as a dogfight weapon, complementing cannon. This reflects
in the widespread use of such weapons, eg the AIM-9J/L with the F-16,
the R.550 Magic/Mirage III/F1 or the Israeli Rafael Shafrir used with
virtually all IAF fighter aircraft, not to speak of the K-13A Atoll or
the AA-8 Aphid used by Warpac air forces.

A guided missile can be broken down into three systems,
guidance and control, warhead and propulsion, all fitted to an
airframe.
Propulsion is usually provided by a solid propellant rocket with a burn
time of the order of seconds, this is adequate for acceleration to
speeds cca 3.5M. The warhead is usually small in weapons of this class,
as it is assumed the missile will detonate either on the target or
within it, warheads are commonly high explosive/fragmentation types .
Most weapons employ a combination of proximity and impact fusing.

The guidance and control systems of the missile occupy it's
nose section, the guidance senses the position of the target and issues
commands to the servoes in the control section which then actuate the
control surfaces to achieve the desired flightpath correction. The vast
majority of operational IR guided missiles employ a canard control
surface/tail stabiliser configuration, the type of canard employed
usually betraying the particular emphasis placed during design, eg the
stabilising fins fore of the canards on the R.550 serve to prevent
stalling at high angles of attack.

The guidance system itself commonly consists of a
window/filter assembly in the nose of the weapon, this serves to select
only particular wavelengths of IR, these then enter an optical
modulation system, a reticle or chopper, which enables a detector
element to receive IR emissions from the target, while rejecting
clutter. The output from the detector is processed by signal detection
electronics which separate target position carrying information from
the
clutter present, a computer then employs proportional navigation to
generate guidance commands.

IR Optical Filters

An optical filter is a device, which, by some particular
mechanism, allows the transmission of some wavelengths, while
suppressing others. The principal reason behind the use of filters in
guidance systems is the necessity to suppress background IR radiation,
such as reflected solar energy, or thermal radiation from the earth's
surface and to enable the guidance to discriminate between various
parts
of the target's signature, as it wouldn't be very helpful to have a
$20,000 missile guide into a ten metre long afterburner plume and
detonate without damaging the target.

Optical filters used in these applications fall into two
broad
groupings, absorption filters and interference filters. Absorption
filters are characterised by wide bandwidths (width of transmitted
band)
and are usually employed to suppress large regions, typically sunlight.
Interference filters can be designed with extremely narrow bandwidths
(less than 0.1 of the wavelength at the band centre) and good
transmittance, they have the further advantage of reflecting unwanted
energy instead of absorbing it.

The physical phenomenon exploited in this instance is
interference, an effect which occurs when we add a wave to it's own
reflection. Consider a series of layers of transparent material, the
layers with alternating refractive indices. If we pass a light wave
through these layers it will be partly reflected at each interface
between layers, alternate interfaces reflecting in and out of phase.
Now
if the wavelength of the wave is four times longer than the thickness
of
the layers, an interesting thing occurs namely the reflections from
successive interfaces are all in phase, leading to a very high
reflectance for that wavelength. Filters employ layers of varying
thicknesses to achieve certain degrees of reflectance for particular
wavelengths.

Precise filters employ up to a hundred layers, each must have
a very accurately defined thickness (for IR less than 1 micrometre), in
order to meet the bandpassing specifications.

Optical Modulators

Probably the most complex individual mechanical assembly in a
missile guidance system is it's optical modulator or reticle. It
performs two extremely important tasks, providing the system with
directional target information and suppressing background IR radiation,
In principle, a reticle is a IR transparent substrate with a particular
pattern of opaque and transparent fields on it's surface.

In operation the reticle is placed between the filter/optics
and detector, it's motion relative to the optics results in the
chopping
of the IR incident on the detector in such a fashion as to enable the
electronics processing the detector's output to separate information on
the target's direction from background images, typically sunlit clouds.

The subject of reticle design is quite complex, to make
things
more difficult, the military has had most information on the subject
classified, however the basic principles may be understood from the
following.

Diagram 3A illustrates a simple rotating reticle for
background suppression. Consider the reticle to be rotating at a
constant rate, then visualise it passing over the image in it's field
of
view, The chopping action will result in different detector outputs for
the point target and for the cloud. The pulses corresponding to the
target (we assume the target is distant enough to be regarded as a
point
source) may then be easily separated from the rippled pulse
corresponding to the cloud by electronic filtering (a narrow bandpass
filter at the pulse frequency), thus enabling the required
discrimination between the target and cloud.

Diagram 3. Target
discriminating
and direction finding reticles.

Reticle A separates target information from
background IR.

Reticle B enables the guidance to find the direction
of the
target.

The third reticle combines the functions of A and B

Actual reticles employ
very fine
patterns, usually with complex wavy or zig-zag fields.

Diagram 3B illustrates a reticle configured for the finding
of
the target's direction. Once again consider a constant rate of rotation
and pass it over the target. A pulse is generated during each rotation
of the reticle, however the instant in time when the pulse commences
depends on the angular position of the target. The time lag or lead of
the pulses or phase carries the information as to the target's angular
position, this information can be extracted via simple electronic means
if we know the position of the reticle as it rotates, which is quite
easily accomplished in practice.

The third reticle in diagram 3. combines the functions of
reticles A and B, providing directional information and background
rejection. The upper half is comprised of opaque-transparent fan shaped
segments, the lower half is semitransparent with a transmittance equal
to the average transmittance of the upper half. When the segmented half
passes over the target, the output will contain a series of pulses and
some varying output given by the back around, when the semitransparent
part passes over the -target the output corresponds to the average
brightness of the target and background. The output would resemble B,
but with bursts of pulses instead of individual pulses.

By electronically filtering out these bursts, we can separate
target information from clutter, the phase of the bursts yields the
angular direction. The radial distance of the target can be found by
examining the amplitudes of the pulses, as the actual image of the
target on the reticle is a circle rather than a point. The width of the
segments on the reticle is smaller than the circle's diameter, if the
circle is near the edge of the reticle a lot of light is passed
through,
if it is near the centre, very little is passed, causing the observed
variation in amplitude. Knowing the angular and radial components of
the
target's direction, we can easily find the X and Y components with
respect to the missile's control axes, a computer can then find the
required control deflection for target interception. It may be apparent
to many a reader that this system cannot provide target direction
information if the reticle axis (missile axis) is pointing directly at
the target, actual operational systems employ complicated mechanical
systems for the rotation and nutation of the optics and reticle to
avoid
this.

The reticle patterns employed are also quite complex and must
be extremely accurate.

An alternate method of determining the target's direction
would involve an array of detector elements, however the electronics
required to separate the target from clutter would be substantially
more
complex.

[Editor's Note 2005: in the nearly quarter century since this
was written we have seen Focal Plane Array IR seekers displace the
reticle based designs described in this article.]

IR Detectors

The detector is a device which converts IR energy into some
electrical signal, which is then processed by the missile's
electronics.
As a device, the detector comprises a piece of semiconductor material
(the photosensitive element), with antireflective and/or filter
coatings
and a reflector, which increases sensitivity by reflecting any IR which
may have passed through the detector back into it.

The two principle types of detector element used are
photoconductive and photovoltaic, the former change their electrical
resistance when illuminated, the latter generate an output voltage on
illumination. An in depth look at the mechanisms responsible for these
effects exceeds the scope of this treatment, but some understanding may
be gained from the following look at solid state physics.

One of the basic conclusions of quantum physics was the fact,
that electrons orbiting an atom may have only certain discrete
energies,
energies other than those corresponding to the given orbits being
forbidden. If we examine the electronic structure of a multielectron
atom, we find the electrons occupying the outermost orbits are the
easiest to remove from the atom, by some external force. If we then
take
a large number of these atoms and begin to move them together, the
orbits begin to interfere with each other with the result, that the
previously sharply defined energy levels begin to smear out leading the
formation of energy bands rather than levels for large numbers of
atoms.

The uppermost two levels, the so called valence band and
conduction band are of the most interest. The electrical resistance of
a
material depends on the number of free electrons in the material, the
more free electrons, the lower the resistance. In terms of energy
bands,
an electron must transition from the lower valence band to the upper
conduction band before it can be available for conduction as a free
electron. The difference in energy between the two bands is called the
energy gap (Eg), an electron in the lower band must receive at least
this amount of energy to transition up and become free for conduction.
One way in which this can happen is when an electron absorbs a photon
of
radiation with an energy hf larger than Eg.

This is basically the effect used in a detector. The energy
gap in the semiconductor materials used is usually less than 1
electronvolt, which is close to the energy of IR photons with
wavelengths shorter than 10 micrometres. Photons which enter the
detector collide with electrons, enabling them to become free and
alter the electrical properties of the detector. However the detector
must be cooled down to around -200 degrees C, as otherwise the thermal
energy of the vibrating atoms would free enough electrons to swamp the
effects of the detected IR. Cooling is provided either by a closed
circuit low temperature refrigerator, Joule Thompson gas expansion
refrigerator or by a thermoelectric refrigerator (early AIM-9J), the
lower the temperature available, the higher the sensitivity of the
detector.

The choice of a detector material depends on the required
sensitivity at a given wavelength, that is given by the temperature of
the target. The majority of the materials used fall into Selenides,
Antimonides and Tellurides, typical IR sensitive materials being
Mercury-Cadmium Telluride (HgCdTe) or Indium Antimonide (InSb).

Early guidance systems were constrained by the lack of
available detector materials to working in the 2.5 micrometre band - as
a result the weapons were easily confused by strong sunlight, cloud
edges or flares and were not effective if launched at a target head on,
as they would rather guide for the plume than the aircraft. Later
systems operate in the more convenient 4 micrometre band, which falls
into a reasonable transmission window and a region where background IR
is fairly low.

New weapons such as the GD Stinger employ combined infra-red
and ultraviolet band sensors (POST seeker), this enables the seeker to
discriminate between targets and countermeasures such as flares, though
it's ability to handle IR jamming systems such as flashing IR beacons
(Cesium lamps) could be questioned. (Further reading: Hudson R.D. -
Infrared System Engineering, Dow R. B.- Fundamentals of Advanced
Missiles).

Heat seeking missile guidance is, after more than twenty five
years of use, anything but obsolete. New technology expands launch
envelopes and extends ranges with each new generation. of weapons. Some
developments in electronics (consider the recent fabrication of 30 x 30
element arrays of HgCdTe on single mounting chips) may lead to
completely different configurations in future weapons, the potential,
for improvements is very large.

Diagram 4. Energy bands of
a pure
semiconductor.

cb- conduction band

Eg- energy gap

vb- valence band

If an electron in the
lower
valence band absorbs a photon hf with an energy greater than the gap
Eg,
it will transition to the upper conduction band and be available for
electrical conduction. Semiconductors sensitive to lower temperatures,
such as Mercury-Cadmium Telluride, have energy gaps around 0.2
electronvolts (eV), those sensitive in the near IR, such as Silicon,
have gaps around 1eV.

Pic.5. The AIM-9L Super
Sidewinder
is a very late model of the widespread Ford Aerospace Sidewinder, which
saw considerable use in the Viet-Nam war. The missile weighs in at 85
kg, has a cruise velocity of Mach 2 and carries an 11.4 kg
fragmentation
warhead. The weapon has an effective range around 20 km and has been
launched from a variety of aircraft, including USMC AH-1T Sea Cobra
attack helos. The launch aircraft in this instance is a TF-18A combat
trainer. The F-18 is configured to carrry two AIM-9s for close-in
combat, these are mounted on wingtip launch rails., The F-18's
integrated cockpit/fire control automatically conditions radar and HUD
modes for the selected weapon and features a number of auto-lock on
modes, including an off-boresight mode allowing the pilot to lock on to
a target in a tight turn. Earlier aircraft such as the F-4 featured
systems like VTAS (Visual Target Acquisition System), where the missile
seekers were cued by a helmet mounted sight.